Solid State Preamp Reviews

In Serinusland, smooth sailing has never been guaranteed. Some months back, for reasons not yet determined, my Stromtank S 2500 Quantum MK II power regenerator (footnote 1) began to hum mechanically—sometimes steadily, sometimes with a pulse—whenever I turned on battery mode. Simultaneously, the Dan D'Agostino Master Systems Momentum HD preamp's power supply began to hum. I could listen through the hum—it wasn't that loud—but silence between tracks was a thing of the past, and rarely was the noise in the right key.

After Stromtank's remote diagnostic and repair procedure failed, Stromtank Chief Engineer/founder Wolfgang Meletzky left Berlin for a previously scheduled visit to D'Agostino headquarters in Arizona. Tests there revealed that Stromtank and this D'Agostino preamp mated perfectly, without generating hum, buzz, pulse, or anything else. With fingers now pointing at the preamp as perhaps defective, I returned it to D'Agostino HQ for a thorough going over. It and a different Stromtank S 2500 Quantum MK II mated perfectly, in blissful silence.

At that point, the two companies arrived at independent decisions. Stromtank decided to send me an S-4000 ProPower MK II so that I could try using it to power everything in my system, including the mono amplifiers, and report my findings in Stereophile. D'Agostino explained that because a replacement for the Momentum HD preamp was in the works—in the form of the Momentum C2, which was introduced at Munich High End 2024—it made no sense to send the Momentum HD back to me.

Instead, D'Agostino President Bill McKiegan asked if I might be interested in writing the first US review of the top-line, three-piece, fully balanced D'Agostino Relentless preamplifier ($149,500, plus $19,500 for the optional digital streaming module), which since its 2021 introduction had only received a single review, in Europe.

Me, review a $150,000 preamp? This was not a kid in a candy store–scale event. This was a kid let loose in a big-assed candy factory–scale event.

My glucose levels spiked. Questions whirled. What new virtues might a cost-no-object, presumably state-of-the-art preamplifier bring to my reference system? Would images be more corporeal? Would the soundstage be wider and deeper, tonal colors more intense? Would bass—already fabulous—be even more solid? Would the Relentless preamp move me closer to a premium-seat-in-a-live-concert experience?

Or would all those possible pluses (and others)—assuming they delivered—be outweighed by previously unperceived weaknesses in my system, now exposed in the blindingly bright light of $149,500 worth of resolution and transparency?

And so began the review. As typically happens with exciting new components, work and fun vied for supremacy until everything was tuned, until I felt the relief that comes from knowing my reference system was passing on sonic dividends—whether all or only some of what the new component offers remains, as always, impossible to know.

What do you get for $149,500? Part 1
The Relentless preamp comes with some intangibles that one cannot count on in high-end hi-fi: a well-designed webpage and a readable, information-packed manual. From these resources, I learned that the Relentless preamplifier's heavily shielded power supply is housed in its own chassis, which is intended to sit between the boxes housing the two audio channels. I learned that the power supply incorporates "internal line conditioning circuitry [that] filters RF noise on the AC power and compensates for asymmetric power waveforms and DC on the mains." Two 150VA toroidal transformers supply power, one to the analog circuitry, the other to the digital and control circuitry.

"These transformers drive an 8A bridge rectifier and 26,400µF of filter capacitance," the D'Agostino website states. The company claims the Relentless preamplifier has "nearly as much power supply capability as many power amplifiers."

A "novel" discrete, differential FET input stage can handle a 30V input signal—compare to the "Red Book" CD full-scale output voltage of 2V—and has an input impedance exceeding 1M ohm. D'Agostino boasts about the preamp's "consistent bias," said to maintain sound quality regardless of component temperature, and "a nearly 30-fold improvement in linearity over conventional designs," which I translate as "big reduction in distortion." The Relentless utilizes four-layer circuit boards, careful segregation of analog circuitry, digital circuitry, and ground planes, and hermetically sealed relays with gold-plated contacts. The signal path is "fully complementary and balanced from the input to the output." The frequency response remains flat up to 120kHz, D'Agostino insists.

The Relentless is the first D'Agostino preamplifier to offer a Digital Streaming Module (DSM), which is to say, a built-in DAC with streaming capability. The DSM is optional of course, and field upgradable. I received it after this review was complete; I will evaluate it in a follow-up review.

The DSM supports PCM up to 32/384 (including MQA) and DSD up to DSD256, both decoded natively. Tidal, Qobuz, and Spotify streaming are supported, and the unit is Roon Ready. It's controlled by an iOS app. (There is no Android app, apparently, at least for now.) The DSM includes S/PDIF coaxial and optical (TosLink), USB-B, Ethernet, and Wi-Fi inputs. When installed, the DSM lives in the power supply chassis, which keeps it away from low-level preamp circuitry.

D'Agostino Master Systems states that "each volume control is constructed using 14 separate metal components [to ensure] the smoothest response and control of the military-grade, high-linearity solid-state switches and discrete precision resistors employed in the volume circuit. There's a different resistor for each volume position, with resistors matched for left and right channels. The bandwidth and transient response of the Relentless Preamplifier is completely unaffected by the volume setting." This point was reiterated later: The volume control has no sweet spot. Speaking of which, the volume control knobs are the smoothest I've ever turned.

Input three can be configured as a theater bypass, allowing a connected surround-sound processor to control the volume of the main speakers. There's an RS-232 control output and a 12V trigger. The power supply has extra DC outputs for some future product, plus two programming ports for future options.

Part 2: Cabinet and remote
On each side of the easy-to-read volume readout in the center of the power supply module's front panel are, on the left side, five buttons: Standby/Power On, Mute, Polarity, Zone 1, and Zone 2—yes, this a two-zone preamplifier, with independent inputs and outputs. On the right are five source-selector buttons, labeled One through Five. It is possible to play the same source to both zones simultaneously, or to play different sources to each zone. In this case, the two volume controls operate independently, one for each zone.

On each preamp's rear panel are five inputs (XLR only), two Zone 1 outputs (both XLR), and one Zone 2 output. The rear panel also includes an IEC power input, an on/off switch, a serial number plate—the usual stuff.

The Relentless preamp's rechargeable, volume-knob–shaped remote control is, or was, a huge, weighty affair—it would make an effective weapon if hurled or catapulted. I won't discuss it further because there's now a new version, which was almost ready to ship when I submitted this review.

Part 3: Talking the walk
Toward the end of a Zoom interview that included the man himself—Chief Designer and cofounder (with Petra D'Agostino) Dan D'Agostino—McKiegan, and Vice President of Engineering Burhan Coskun, Coskun outlined what he considered most important about the Relentless preamp's design and dividends:

1. The Relentless has ultralow distortion and operates discretely to ensure exceptional performance and stability across various system configurations.

2. Each channel has its own power transformer. Audio signals and power travel through gold pins embedded in the feet of the top and middle chassis.

3. The noisefloor is extremely low, and the dynamic range is wide.

4. All this adds up to a transparent soundstage with exceptional detail and resolution.

There's more to say, and far more was said before that summary arrived. According to Dan, the Relentless is something totally new and different for D'Agostino. My reference Momentum HD preamplifier "has almost nothing to do technically with the Relentless preamplifier. Nonetheless, the ideas that we learned while designing the HD spurred us to design the Relentless." The difference between the two is "all about how much space you like and how much information you like that's way down in the black part of the audioband.

"I don't know about you, but I want to hear the fine-grained detail that's way down there that most equipment doesn't reveal, particularly if you run a DAC right into the amplifier. I want to hear everything. If there's dirt in the background, if there's hum in the background, if there's somebody scraping their feet in the other room, I want to hear all that. Because in real-life music-making, that's a part of the feeling. Totally. In addition, don't you want your soundstage to be as big as an orchestra's?"

I asked D'Agostino to explain the implications of each of the technical advances he'd incorporated into the preamp. He replied, "I react to differences in sound. When I had Krell, I reacted to differences in technology; this company is based in sound only, and sometimes technology advances our quest.

"If an electrical improvement doesn't translate into better sound, we don't use it. Even if it's the greatest advancement since sliced bread, it's not going in the unit if it doesn't sound better. Burhan has been with me for almost six years, and his audio knowledge comes directly from me. He's grown into a really good audio designer. He knows that when we listen to something, I can grow impatient, because from the first note, I know if the design is good or bad. Together, we bounce ideas back and forth and make something that sounds better."

Some of D'Agostino's circuits, including those in the Relentless amplifier, are based in other technological realms—"in measurement and instrumentation amplifiers that have nothing to do with audio," Dan said. "The whole front end is based on that. Once we adapted the technology to work in audio—it took a year before we even considered putting it into production—it sounded amazing. In developing the Relentless preamp, we took what we learned from experimentation and pushed it as far as we could push it. I'm not gonna tell you that in a year or two years that we might not say, hey, wow, I see something better. But right now, at this juncture in my life, I don't think we can make anything better."

McKiegan noted that a preamp's three primary signal circuitry functions are its input stage, volume control stage, and output stage. The Progression preamplifier handles those functions with two boards. The just-discontinued Momentum HD preamp used six boards, three for the left channel and three for the right.

The Relentless also uses three boards per channel, but there is a completely separate section for its Zone 2 preamplifier. That makes 12 boards total, plus additional boards for purposes other than carrying signal.

"As the real estate gets larger, there's more availability for circuit sophistication," McKiegan said. "As we climb the ladder, each step allows for the greater audio technology that the guys are able to put in the boxes. In the Relentless, all sections within each chassis are on separate boards and platforms. In addition, the entire digital control module, volume control, logic board—all that stuff—are in the center box, and audio signals are isolated from other signals. In every respect, there's so much more isolation."

"Bigger transformers create less circuit noise because their cores do not radiate the noise that smaller transformers do when pushed close to their limits," D'Agostino explained. "In preamps, the purpose of bigger transformers is not to create more power; instead, they run cooler and wider with less noise." The Relentless has separate transformers for the digital/control and analog signal circuitry.

When it comes to power products, accessories, and tweaks, every designer I've encountered has a bottom-line belief system. I hear benefits from Wilson Audio Pedestals and HRS vibration control devices, but Dan D'Agostino is of a different mindset. "I know a lot of people talk about vibration in audio products, and you see those ridiculous bricks sitting on top of somebody's product," he said. "We design our products so that vibration is not really going to affect them at all. If a product needs those things, then the designer didn't really do a good job in the beginning."

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Footnote 1: I reviewed the Stromtank S 1000.